Export 2D view of objects
You can use an orthographic camera. That way perspective has no effect and all of the objects will be the same size in all the images. Since all the camera settings are still available png files are already the default output file type.

To get an orthographic camera, simply click on your camera and go to the camera tab in the properties window. At the top, click orthographic, and change the scale until all of the parts fit in to the frame.

If you change the orthographic scale at any point the objects will no longer be the same size, so make sure you test all of the angles to make sure there are no problems before rendering.